Layer group mode "Pass-through" does not get loaded from project file.

[ficklef0x]

Description of the problem

When selecting a mode for a layer group, the option "Pass-through" can be selected, but not loaded file, it has to be set every time you load the project file for each layer group.
You can export the file to any format with the layer mode applied.

I think it might have something to do with being unable to set the render mode to pass-through on normal layers, idk.

Basic system details

MyPaint version: [2.0.1+git.478280f2]
Operating system: Windows 10 Pro x64, 21H2, 19044.186

Steps to reproduce

1. Start MyPaint
2. Create a layer group with a few layers in it and set their modes.
3. Set the layer group mode to "Pass-through."
4. Save the file.
5. Reload the file.
6. The layer group mode is reset to "Pigment." You have to re-apply the pass-through mode for each layer group manually.

Backtraces or error messages

There are no error messages.